The number of people killed after Sunni Islamist militants overran the Iraqi city of Mosul earlier this week may run into the hundreds, UN human rights spokesman Rupert Colville said on Friday.

He said his office had reports the killings included the execution of 17 civilians working for the police and a court employee in central Mosul.

Four women had killed themselves after being raped, 16 Jordanians had been kidnapped, and prisoners released by the militants had been looking to exact revenge on those responsible for their incarceration, he said.

"We've also had reports suggesting that the government forces have also committed excesses, in particular the shelling of civilian areas on 6 and 8 June," he said. "There are claims that up to 30 civilians may have been killed."